His Holiness is a style and form of address (in the variant form Your Holiness) for some supreme religious leaders, originally and most notably the Pope.
His Holiness (Latin: Sanctitas) is the official style used to address the Roman Catholic Pope. In the Eastern Orthodox Church, the Ecumenical Patriarch of Constantinople has the title of His All-Holiness (abbreviation HAH).
The term is sometimes abbreviated to "HH" or "H.H." when confusion with "His/Her Highness" is unlikely. The associated form of address is "Your Holiness".
It is also used for certain other Eastern Patriarchs, notably those who head a church or rite which recognizes neither Rome's nor Constantinople's primacy, such as any Catholicos of the East.
The English language honorific "His Holiness", and as female version "Her Holiness", has commonly been used for religious leaders from other traditions, including Buddhism (notably for the Dalai Lama), Shinto and in Ahmadiyya Islam for the Caliph.